1. Field of the Invention
This invention relates generally to magnetic recording and/or reproducing apparatus, such as, video tape recording and/or reproducing apparatus (VTR), and more particularly is directed to improvements in a tension control device for such apparatus.
2. Description of the Prior Art
Existing video tape recording and/or reproducing apparatus generally comprise a tape guide drum having one or more rotary magnetic heads associated therewith to record or reproduce video signals on a magnetic tape which is wound on supply and take-up reels with the tape between such reels being wrapped helically about at least a portion of the circumferential surface of the drum and being driven by a cooperating capstan and pinch roller and by suitable rotation of the take-up reel. It will be apparent that, during a recording operation of the foregoing apparatus, each rotary magnetic head will travel obliquely across the tape at an angle to the longitudinal direction of the latter that is determined primarily by the helical path of the tape on the guide drum, the rotational speed of the rotary magnetic heads and the longitudinal speed at which the tape is driven by the capstan, whereby to record the video signals in successive parallel record tracks that are similarly obliquely related to the tape. When reproducing video signals previously recorded in the oblique record tracks, either by means of the same or a similar recording and/or reproducing apparatus, the rotational speed of the rotary magnetic heads and the speed at which the tape is longitudinally driven are controlled by suitable servo systems so that each rotary head, as it commences a scanning movement obliquely across the tape, will be aligned with a record track. If the magnetic tape is considered to be dimensionally stable, and assuming that the previously mentioned factors determining the angle of the oblique record tracks are the same for the reproducing operation as for the recording operation, then each rotary magnetic head will scan precisely along an oblique record track for reproducing the video signals recorded therein. However, the magnetic tape, being usually formed of a plastic film with a magnetic coating thereon, is not dimensionally stable, that is, its length may be varied by changes in temperature, humidity and tension. It will be apparent that, if the length of the tape changes between the recording and reproducing operations, then the paths along which the rotary magnetic heads scan the tape during the reproducing operation will be at a small angle to the oblique record tracks in which the video signals are recorded. Therefore, even though each head is aligned with a record track at the commencement of its scanning movement obliquely across the tape, the head will deviate from that record track in the course of the scanning movement and the reproduced video signals will contain guard band noises which result in a characteristic "snow" or disturbance in the picture or image displayed by a monitor or other image display device.
It is known to eliminate the above mentioned guard band noises during reproducing operations by suitably varying the tension in the tape so as to compensate for any changes in the longitudinal dimensions thereof that may have occured between the recording and reproducing operations. In existing video tape recording and/or reproducing apparatus, the tape is usually tensioned during recording and reproducing operations by means of a brake which resists rotation of the supply reel, with the braking force applied by such brake being varied in accordance with the tension in a run of the tape between the guide drum and the supply reel.
For example, as disclosed in U.S. Pat. No. 3,833,921, issued Sept. 3, 1974, and having a common assignee herewith, a pivotally mounted tension control arm may be provided with a pin at its free end pressed against a run of the tape between the guide drum and the supply reel by means of a spring acting on the arm. Further, the tension control arm is connected to a band brake which extends around a brake drum on a rotatable supply reel support member to resist rotation of the latter, and hence of a supply reel rotatably coupled therewith. It will be apparent that, in the foregoing arrngement, the position of the tension control arm is at all times determined by the force of the spring urging such arm to turn in one direction and by the tension in the tape run engaged by the pin on the tension control arm for resisting turning of the latter in that direction. The band brake is arranged so that a movement of the tension control arm in one direction in response to a decrease in the detected tape tension increases the braking force so as to restore the tape tension to a predetermined value, whereas a movement of the tension control arm in the opposite direction in response to an increase in the detected tape tension decreases the braking force for again restoring the tape tension to the predetermined value. Thus, the described arrangement tends to maintain a constant tape tension which is determined by the force of spring acting on the tension control arm. In order to permit adjustment of the tape tension which is to be maintained, the existing apparatus connects one end of the spring to the tension control arm while the opposite end of the spring is connected to an angularly adjustable anchor arm. Therefore, in theory, when guard band noises appear in the reproduced video signals by reason of changes in the longitudinal dimensions of the tape between recording and reproducing operations, the anchor arm for the spring can be angularly adjusted to change the spring force, and hence the tape tension, in the sense required for returning the tape to its longitudinal dimensions during recording and thereby eliminating the guard band noises.
However, it has been found that, in practice, the necessarily fine adjustment of the tape tension cannot be easily achieved by angular adjustment of the anchor arm which is normally locked in a selected position. The foregoing difficulty results from the fact that, when the lock retaining the anchor arm in a selected position is released to permit angular adjustment of the anchor arm, the full force of the associated spring tends to turn the anchor arm in the direction to reduce the spring force. Therefore, the adjustment of the anchor arm has to be effected against the full force of the associated spring which interferes with the precise relocation of the anchor arm to achieve the required precise adjustment of the tape tension.